What is the trick to riding on sand?

Fatter tires and stand up. If you stay seated, the front tire will not be able to carry its weight and basically float, making it really hard to control. Growing up on the beach in Hawaii, I have had my fair share of riding in sand and found that the wet sand that the ocean just saturated is 30 times easier to ride on than dry sand.

I have plenty of experiance in insanely soft sand. Before you leave : 29er, wide tyres, low pressure. BUT Low preasure costs when you get back onto hard stuff, as do insanely wide tyres. I set the bike up to go well on the hard stuff, and pay the price on the soft sections.

On the track : Riding style is Weight Back, Very high cadence, great balance and steer with weight shifts rather than handlebar movement.

It VERY quickly becomes faster, easier and more efficent to dismount and run/carry bike. The only reason to ride soft sand to prove you can and get fit fast. If the tracks are mostly soft sand (and you are like me - out to have fun), find better tracks, as it's not much fun after the novelty wears off.

One big advantage, it's a soft landing when it all goes wrong

I have personally ridden and raced on many sandy courses and trails (e.g. Moab) and the best advice is to shift your weight back, relax and think about guiding rather than steering the bike.

You will never be able to ride in a perfectly straight line, so get over it or avoid sand. Rather you need to let the bike move and shift around under you. You will need to stay loose and relaxed while making sure to have a firm grip. You can't force the bike or it will skid out. Rather your job is to look ahead and guide the bike in the general direction you want to go. You also need to avoid sudden jarring movements. If you have ridden in dry snow, the feeling is quite similar.

I know some answers said avoid excessive speed (which is good advice for riders new to sand) but I will say it is also possible to scream along on sand. I have hit deep and long sandy sections at full speed before (e.g. 50km/hr) and been fine. Sure the bike moved around a bit, but as long as I don't panic (and tighten up), kept my weight back and guided the bike I was fine.

If you are riding uphill or on the flats try to keep steady power on the pedals as you have less traction to work with. Some find a faster or slower pedal cadence easier - try both and take your pick. Also make sure to conserve momentum, if you slow down to much you won't be going anywhere. Finally, don't get discouraged, riding on sand is a learned skill and always work to

Lastly, there are also technical aspects (e.g. tire pressure) that have already been covered (heltonbiker)

Riding on sand is pretty easy once you get the hang of it, keep yourself heavy on the front of the bike through technical dh sections and over the back on a low gear on climbs, when riding on sand around corners (especially deep sandy sections) avoid leaning into the corners too much and don't be afraid to counter steer with the bars, just don't do it too quick or you will be jerked off, try not to ride the brakes too much especially when going around corners or you could easily get bogged, stay loose and ride the smoothest, straightest line.

Try not to pedal down. Instead, if you use clips, push pedals front and back. Also don't hold the handlebar very tightly. Relax, let it go where it wants to go.

One additional trick which the existing answers didn't mention: you might want to shift your weight constantly back-and-forth from front wheel to back wheel. Maybe synchronize it to rotating your cranks - do one front-back-front cycle per one turn of your pedals.

A pseudo-physical explanation for this is: traction is better when there is more weight on the wheel. However, more weight also means the wheel sinks more. If you constantly shift your weight, you get the best of both - good traction when your weight is on the back wheel, and each of your wheels gets out of the sand when there is less weight on it.